General rule: You can stall any aircraft in any attitude and at any speed*. Stalling has no direct relationship to the horizon, only to the relative airflow.
A non-symmetrical (cambered) aerofoil (as found on most wings) will stall at a lesser negative angle of attack than it will at a positive AoA. Any wing can be stalled negatively as well as positively, however due to perfomance, limitations and normal usage of light aircraft, negative G stalls are unlikely to be encountered unless you are doing something very unusual.
The fact that in your example an aircraft flick rolls with its nose below the horizon does not automatically equal a negative stall - that depends on wether from that attitude the pilot pulls up or pushes forward to induce an accelerated stall and flick roll. Competition aerobatic aircraft will often have symmetrical aerofoils and large control authority.
* Small print: Zero airspeed or zero G are special situations, and at high speed structural limits may be exceeded before an aerodynamic stall occurs. Available control authority my restrict the ability to induce a stall in some circumstances.
Last edited by FRFO; 6th Aug 2012 at 13:33.